1. Field of the Invention
The present invention relates to a distortion compensation apparatus and particularly to a distortion compensation apparatus and a distortion compensation method which are applicable to a high-frequency power amplifier for transmission used in a portable phone.
2. Description of Related Art
As the communication has come to have a higher speed and a larger capacity in recent years, stricter linearity has been required for a transmission power amplifier in a digital wireless communication apparatus. It has simultaneously causes a situation that improvements in power efficiency in a power amplifier are prevented.
Meanwhile, the continuous communication time of a digital portable phone which has already spread in the general market has been steadily elongated. Therefore, in introduction of a new digital wireless communication apparatus into the market, the use time cannot be neglected, from the viewpoint of competition among products. Today, the movement of introducing a distortion compensation technique to improve the efficiency has become active.
In this technique, however, its circuit scale is too huge to realize it in a portable phone whose advantage exists its small size and light weight. In addition, due to characteristics of a portable terminal, the environment in which the terminal is used changes so greatly that distortion compensation necessitates adaptive distortion compensation which follows the environmental change. This has become a very important problem, as well as downsizing. For a distortion compensation apparatus of this kind, a pre-distortion technique provided with a compensation means having a characteristic opposite to the distortion of a power amplifier has been known.
As pre-distortion techniques of this kind, there are several reports about a technique which adopts pre-distortion, a technique which adopts feed-forward, and the like. The following will explain examples of conventional adaptive distortion compensation apparatuses using the pre-distortion technique.
A first example of a conventional structure is, for example, 1992. European Microwave Conference, Vol. 22, pp. 1125–pp. 1130, “Power Amplifier Adaptive Linearization Using Predistortion with Polynomial.” FIG. 1 shows a block diagram of the example disclosed in this reference.
In FIG. 1, where the non-linear input/output characteristic of a power amplifier (PA) 114 whose distortion should be compensated for is expressed as Vout=A(Vin), an in-phase signal I and an orthogonal signal Q of an input base band inputted from an input terminal 111 are subjected to calculation using a function H (I, Q) which linearizes A (Vin), in a linearization comparator circuit 112. I′ and Q′ signals obtained as a result are supplied to digital/analogue conversion circuit (D/A) 113, and are converted into analogue signals. At the same time, they are converted into signals of a high frequency band, and are inputted to the power amplifier 114. The output Vout of the power amplifier 114 is outputted from an output terminal 115, and is also supplied to a demodulation circuit 116. The demodulation circuit 116 generates If and Qf signals into which the output signal Vout is converted into signals of a base band.
Further, to perform adaptive compensation in response to a temperature change, the linearization comparator circuit 112 compares I and Q signals with If and Qf signals and adjusts a constant included in the function H for linearization such that the differences among them become zero. Until the differences become zero, this operation is repeated so that the constant included in the function H (I, Q) is finally determined to an optimal value.
An example of another conventional structure is, for example, IEEE Transaction on Vehicalar Technologies, Vol. 43, No. 2, May 1994, pp.323–pp.332. “Adaptive Linearization Using Predistortion”. FIG. 2 shows a block diagram described in this reference. With respect to input signals I and Q inputted from an input terminal 121, a conversion table 124 such as a memory or the like is accessed thereby to perform data conversion, to obtain data I′ and Q′ which are capable of linearizing the power amplifier 126. The data are converted into analogue signals by a D/A converter 125 and are then inputted to the power amplifier 126. The output Vout thereof is detected and converted into a signal of a base band by a demodulation circuit 128, to obtain signals If and Qf. Further, to perform adaptive compensation, differences en between the input signals I and Q and the detection signals If and Qf are obtained by a subtracter 122. An address generation section 123 adjusts addresses in the conversion table 124 such that the differences en become zero. Specifically, the address generation section 123 repeats adjustment of the addresses until the differences en correctly become zero. Thus, address values for accessing the conversion table 124 are optimized. Further, Vin, which is obtained by converting data I′ and Q′ outputted from the conversion table 124 into analogue data by the D/A converter 125, is inputted to the power amplifier 126, and the output Vout thereof is guided from an output terminal 127.
In the conventional structures described above, the constant included in a linearization function or addresses for accessing a linearization table are optimized. In any examples, however, the output of the power amplifier is converted into a base band, so a demodulator is required. In general, this demodulator is of orthogonal demodulation, and therefore, its circuit scale is huge.